For a long trip, I will always check a bag. I like to be able to bring all of my cosmetics and lotions and liquids. That said, the carry on will still hold some essentials.
Clothes for the next day in case your luggage gets lost.
Snacks for the plane. One bag of extra small peanuts or pretzels ain’t gonna do it.
Gum for chewing in the ascent/descent to keep ears for popping.
Any electronics or cameras – I like to keep these on my person.